1. Field of the Invention
The present invention relates to a biaxially oriented film of a polyoxymethylene polymer having the main part of the each of the main chains thereof composed of repeating units of the oxymethylene group, --CH.sub.2 --O--, in which said biaxially oriented film is highly oriented and crystallized and possesses high rigidity and high dimensional stability.
2. Description of the Prior Art
In recent years, owing to the growing trend of electronics devices toward the addition of density, reduction of size, and acceleration of signal processing speed, the providing of films which are to be used for such magnetic recording media as video tapes, audio tapes, and floppy discs of minimum thicknesses and with ensured high dimensional stability, are being urged with increasing enthusiasm. Fulfilment of these requirements necessitates the development of a high-performance film which possesses outstanding rigidity and dimensional stability. The present invention has been originated due to the appreciation of a polyoxymethylene polymer film as a promising film that may satisfy these requirements. One of the objects of the present invention is to provide a biaxially oriented polyoxymethylene polymer film which possesses high rigidity and high dimensional stability.
The polyoxymethylene polymer film is generally formed by melt pressing or melt extruding the film or sheet form of the polymer and subsequently quenching the pressed or extruded film or sheet. The film obtained by this type method is opaque or translucent and further, is deficient in mechanical properties. Various means of improving the transparency and the mechanical property of this film have heretofore been tried. The technical advance in this field has reached a point where there are produced films with the transparency and mechanical properties thereof having been improved to an appreciable extent. Even though handling of the polyoxymethylene polymer film is extremely more difficult as compared with conventional films, the outstanding properties inherent in this film, i.e. the excellent rigidity and dimensional stability possessed by this film as a biaxially oriented film, have not eluded attention.
It is held that one of the causes for the difficulty experienced in the handling of the polyoxymethylene polymer film resides in the relatively large spherulites which are present in the crystalline structure. Japanese Patent Publication No. 21994/1965 discloses a method for the production of a polyoxymethylene polymer film with improved transparency and mechanical property by the steps of rolling a crystallized polyoxymethylene film translucent or opaque owing to the presence of sphelurites at a rolling temperature of not higher than 120.degree. C. until the film becomes substantially transparent and substantially ceases to show any detectable sphelurite under a microscope in visible light and subsequently stretching the rolled film at a temperature in the range of 120.degree. to 180.degree. C. by a ratio of not less than 25% in at least one axial direction.
This method is characterized in that the transparency of the film is improved and the orientation of the crystals of the film is facilitated by rolling the film. As a result, the film obtained by this method is improved in transparency and mechanical property over the film obtained by a conventional method.
Generally, the rigidity of a film is said to be proportional to the modulus of elasticity of the film. To be specific, the rigidity increases in proportion as the modulus of elasticity increases. In the case of a polyoxymethylene polymer film, the highest modulus of elasticity indicated in the working examples of the prior art mentioned above is 530 kg/mm.sup.2, whereas the modulus of elasticity in the direction perpendicular to the direction in which the aforementioned highest modulus is manifested is reported to be about 420 kg/mm.sup.2. The improvement of rigidity to the extent mentioned above is far short of being sufficient for a high-performance film of high rigidity. It is not the type of improvement which the present invention expects to achieve. This prior publication does not disclose the improvement of the possibility of this magnitude being further increased or of a device or means which is capable of permitting this improvement increase.
In the case of the polyoxymethylene polymer film, it has been held that it is extremely difficult to have the crystals of the film oriented to an appreciably high degree by any conventional means of orientation, owing to the presence of spherulites in the crystalline structure mentioned above. The aforementioned prior invention has realized the orientation of crystals of the film to an appreciable extent. The properties which are acquired by the film produced by the prior invention, however, hardly reach the level that the present invention expects to obtain. These properties have been heretofore resignedly been accepted as the maximum levels of performance which could be attained by the polyoxymethylene polymer film used as a biaxially oriented film.